Visiting Harrow in north-west London, Mr Blair insisted Labour still had "every chance of succeeding" in the general election.

Questioned about whether Mr Brown had failed in his time at Number 10, he said: "No I don't think he's failed at all."

Asked about the prospect that the party could come third in the share of the vote on May 6, the former PM replied: "I don't believe that will happen. I believe Labour has every chance of succeeding."

"I think the strange thing from this campaign - because obviously I am not running for office any more, you look at it a bit from the outside - is that there is all the debates and who is up and who is down and the polls and all the rest of it, but when you come to a clinic like this here, which is a fantastic, modern clinic, it is a new way of working, it is not just a new building, and the real question for the future, for the country is, is this going to be the way we are going to go or not?"

As he had his blood pressure taken at a polyclinic in the Labour-held constituency, Mr Blair joked: "The question will be, is it better or worse than it used to be?"

Mr Blair has been largely absent from Labour's campaign so far, aside from a brief appearance at his Segdefield constituency last month. His appearance today comes as Mr Brown stuggles to make an impact on polls which indicate he could come third in vote share on May 6.

Speaking to reporters in Derby, Liberal Democrat leader Nick Clegg said Mr Blair's appearance was a sign of "desperation" in Labour ranks.

He said: "I think it's a measure of some desperation, wheeling out the golden oldies in the Labour Party to try to help out Gordon Brown in his hour of need."

During last night's debate, watched by eight million people, Mr Brown admitted for the first time that the Tory leader is likely to win.

In his closing statement, Mr Brown said: "I know that if things stay as they are, perhaps in eight days' time David Cameron, perhaps supported by Nick Clegg, would be in office.

"But I have the duty of telling you this evening that while we have policies for the future, the Conservatives would put the recovery immediately at risk with an emergency Budget."

His remarks came after the Conservative leader delivered his most effective denunciation of Labour’s 13 years in power while articulating a powerful vision for Britain.

A combined series of snap polls taken after the debate declared Mr Cameron as the clear victor on 38 per cent, Lib Dem leader Mr Clegg on 32 per cent, while Mr Brown languished in last on 26 per cent.

Ahead of next week’s general election, Mr Cameron came close to accusing Mr Brown of being a liar and said the “desperate” Prime Minister had left the country in a “complete mess”.

Mr Brown attempted to fight back by warning that Conservative economic policies risked returning Britain to the 1930s. He staunchly defended a series of Labour policies but was accused of seeking to “frighten” voters.

The debate, aired by the BBC, was the most confrontational and tense of the three exchanges and paved the way for the last week of campaigning in the closest election in recent times.

Recognising the extent of the challenge faced by Labour to win back support, the Prime Minister told a group of supporters afterwards: "From now until next Thursday we have got to campaign like we have never campaigned before."

By contrast, minutes earlier his rival Mr Cameron told a jubilant rally: "We've got just six and a half days to go between now and polling day.

"Don't waste one minute, don't waste one hour, this country is crying out for change, we need to explain to people the only way to get it is a Conservative government."

After becoming the star of the past two debates, Nick Clegg frequently found himself marginalised in the debate as the two main party leaders attacked one another.

He appeared weak on policy detail during several exchanges and his portrayal as an outsider may have lost its novelty value.

Speaking afterwards, Mr Clegg urged voters not to be frightened by the other parties out of voting for "real change".

"This is your election, this is your country," he said. "When you go to vote next week, choose the future you really want."

During the debate, Mr Cameron said that voters needed a “clean break” from Labour. In the wake of the “bigot-gate” controversy, Mr Brown admitted that he did “not get it all right” but floundered as he attempted to defend his record.

“There’s a lot to this job,” the Prime Minister said. “And as you saw yesterday, I do not get all of it right. But I do know how to run the economy in the good times and in the bad.”

His performance will add to growing Labour fears that his party is now on course to come third in the election – the worst result for a governing party in a generation.

Last night, there were allegations that senior Labour aides had conceded they could not win the election.

The Prime Minister’s repeated insistence that he was the only leader with the experience to lead the economic recovery failed to convince viewers.

Last night’s final debate in Birmingham, on the theme of economic affairs was expected to be Mr Brown’s strongest performance.

However, the first polls, taken within minutes of the debate ending, again rated Mr Brown as the worst performer. YouGov/The Sun had Mr Cameron on 41 per cent, Mr Clegg on 32 per cent and Mr Brown on 25 per cent.

A second poll by Comres/ITV News had Mr Cameron on 35 per cent, Mr Clegg on 33 per cent and Mr Brown trailing on 26 per cent.

The parties believe that millions of undecided voters were waiting until the final debate before deciding how to cast their votes. Many will vote by post over the bank holiday weekend.

Mr Cameron will now hope that he can persuade wavering Liberal Democrat voters to switch to the Conservatives in the final days of intense campaigning.

The Tories today unveiled a "contract" being sent to millions of households in target seats in a bid to capitalise on the "momentum" generated by Mr Cameron's performance in the final televised debate.

It sets out 16 key pledges with a message from the party leader to voters saying: "If we don't deliver our side of the bargain, vote us out in five years' time."

Labour is hoping to inject fresh energy into its campaign today with former PM Tony Blair expected to enter the fray. He is expected to visit a constituency in London ahead of other visits across Britain next week.

Mr Cameron used the 90-minute debate to seek to undermine Labour’s 13-year record. He warned viewers: “You are hearing desperate stuff from someone who is in a desperate state.

“The Prime Minister and this Government have left our country in such a desperate state.” He urged voters to “look” at Labour’s record. He accused Mr Brown of increasing taxes “something like 178 times”.

He drew attention to Mr Brown’s previous misleading comments about increasing defence spending. He also said that Labour had set up the “whole economy … to serve the banks” which culminated in the knighting of Fred Goodwin, the former chief executive of Royal Bank of Scotland.

He said: “It was this Government which gave this man a knighthood for services to banking. He very nearly brought down the entire economy.” Mr Cameron warned that anyone earning more than £20,000 was now a “target for Labour”, after Mr Brown said he was increasing National Insurance.

Viewers were also reminded that Mr Brown had scrapped the 10p income tax band, which hit low earners, and only increased the state pension by 75p during one year.

Mr Brown attempted to fight back by warning about the “ideological” drive of the Conservative party. He attempted to focus on Conservative plans to cut inheritance tax and cut the child tax credit for middle income earners.

He accused the Conservatives of “living in the age of the 1980s and 1990s” and warned of the risk of a “double-dip recession”.

The debate took place after the Institute of Fiscal Studies, an economic forecaster, released an analysis which said that none of the parties had come “anywhere close” to disclosing their plans for dramatic public sector cutbacks. The institute said that there was a black-hole of up to £52 billion in the parties’ economic plans.

Yesterday, an economist disclosed that the Bank of England governor had told him over lunch recently that whoever won the election would eventually be out of office for a generation because of the unpopularity of the tough decisions they would have to take. The Prime Minister refused to concede that further public sector cuts would be necessary under his “deficit reduction plan”.

However, Mr Cameron said: “There are going to have to be cuts and we need to be frank about this. I will do everything I can to protect the front line services.”

The Liberal Democrat leader also conceded that further cutbacks were in the pipeline. He said there was a “huge black hole” in the public finances. He said we have got to “get the politicians working together on this … politicians putting the country before themselves.”

Mr Clegg, although providing another polished performance, struggled when pushed to defend Liberal Democrat policies to offer illegal immigrants an amnesty and for Britain to join the euro. At one point, Mr Clegg was reprimanded by David Dimbleby, the BBC’s moderator, for simply repeating his answers as the debate progressed.

The party leaders were asked one question about immigration but Mr Brown’s comments about Gillian Duffy, the Rochdale widow he accused of being bigoted on Wednesday after she questioned the influx of eastern European migrants, were not raised directly.

Mr Cameron turned his fire on the Liberal Democrat leader and attacked plans for an amnesty for illegal immigrants. The Conservative leader said the policy was a “complete mistake” and “profoundly misguided”.

Mr Clegg insisted he appreciated voters’ concerns about immigration. However, he said that politicians needed to “get real” about the radical action needed to tackle the problem.

Despite Mr Cameron’s success in the debate, the Conservatives still have a major problem in tackling Mr Clegg’s popularity in the next week.

The Tories are campaigning hard in previously safe Labour seats as they may now fail to seize constituencies currently held by the Liberal Democrats.

In the immediate aftermath of the debate, George Osborne, the head of the Conservative’s election campaign, said that the Conservative leader had showed he is “ready to be Prime Minister”.

The Tories claimed Alistair Campbell had been overheard remarking “We’ve had it” after the debate, although it later emerged he may have been referring to Burnley, the football team he supports.